These are juveniles, from the white heads. They remain in this plumage for 2-3 years before gradually becoming adults.
During this time they wander about quite a distance, these are probably from breeding grounds in the South China Sea, the Xisha (Paracel) or possibly Dongsha Islands, and reach Hong Kong annually, usually from April to August. Occasionally they may stay in the Pearl River Delta area for some time, up to 6 months or more.

As Kevin mentions, there was a pair at Sai Kung last summer 2014 and at least one in August 2013 also. Whether they are the same birds can probably only be guessed at.

Quote:

原帖由 wgeoff 於 8/07/2015 06:52 發表
These are juveniles, from the white heads. They remain in this plumage for 2-3 years before gradually becoming adults.
During this time they wander about quite a distance, these are probably from bree ...

Hi, I want to ask if this is a Christmas Island Frigatebird (白腹), rather than a Lesser Frigatebird (白斑)? Because refer to "Birds of South China and HK", it seemed like Christmas Island juvenile.
Please help to clarify,Thank You.